Immensely problematic though Afghanistan has become, Pakistan is emerging as a far bigger, potentially more dangerous challenge to western security interests, officials and analysts in Kabul say. The west's central dilemma is how to obtain Islamabad's full support in "degrading" al-Qaida, Taliban and other militant Islamist groups on both sides of the Durand line – the British-designated de facto Afghan-Pakistani border – without fatally undermining the Pakistani government's legitimacy in the eyes of its own people.

"Pakistan is the big coming problem," one analyst here said. "The US and Nato countries have to convince the Pakistanis that they are not going to cut and run in Afghanistan [as happened after Soviet troops withdrew in 1989 and again, arguably, after the Taliban regime was overthrown in 2001].

"Three-quarters of Pakistan's borders are contested, by the Iranians in the west, along the Durand line, by India in Kashmir, by China. Pakistan would prefer an unstable Afghanistan to a hostile one. It worries what India might do there. If we want their help, we have to be able to offer Pakistan geopolitical stability," the analyst said.

Speaking before Barack Obama unveiled his revamped Af-Pak strategy last week, US officials described Pakistani co-operation as essential to achieving western objectives. "We can't succeed [in Afghanistan] without Pakistan," one official said. "And if you don't win in Afghanistan, then Pakistan will automatically be imperilled, and that will make Afghanistan look like child's play."

Yet Obama's speech was notable for its vagueness about Pakistan's role. "We will strengthen Pakistan's capacity to target those groups that threaten our countries, and have made it clear that we cannot tolerate a safe haven for terrorists," he said.

Obama gave no new indications as to how this might be achieved, beyond the additional economic and development aid and intelligence assistance already offered, or what the US would do if its wishes were ignored.

Washington is ostensibly worried that public admonitions or hectoring could backfire. But General James Jones, Obama's national security adviser, reportedly exhibited no such inhibitions during a private visit to Islamabad last month. Jones is said to have warned that if Pakistan did not deliver, the US might be impelled to use "any means at its disposal" to secure the border region. This implied a threatening escalation in a country that already feels its sovereignty is under assault from American drone attacks.

The area in question, known historically as Pashtunistan, was deliberately divided by Henry Durand and the British in 1893. It is home to about 15 million Pashtuns on the Afghan side and 28 million on the Pakistan side. They do not see themselves as belonging to either state; nearly all the Taliban forces come from there. It is the quintessential "ungoverned space".

Yet despite all this, western officials say Pakistani leaders, preoccupied with the strategic challenge posed by their old enemy, India, have still to make a "strategic shift" away from the Afghan Taliban, who it sponsored in the 1990s.

In particular, tougher Pakistani action is sought against the so-called Quetta shura, the Taliban's ideological headquarters, and militant groups bent on provoking confrontation with India.

"For the Pakistanis, the Afghan Taliban are still an insurance policy. We have to persuade them that the Taliban are a threat, not an ally, and that the result of [the US-led surge] will be stable, friendly Afghanistan," one official said. Trying to provide such reassurance, Obama has offered to facilitate an India-Pakistan rapprochement, a demarche that has met with a deafening silence on both sides.

One possible consequence, should the US over-reach, could be the collapse of President Asif Ali Zardari's weak and unpopular administration. Conspiracy theorists say this may be Washington's intention; that it may prefer to deal with the Pakistani military, as during the era of former strongman General Pervez Musharraf.

Maleeha Lodhi, a Pakistani former diplomat and influential columnist, says frenetic western cajoling is fraught with risks for Pakistan, where anti-American sentiment is widespread and extremists are pursuing a daily campaign of terror attacks to destabilise the country.

"Intensified fighting in Afghanistan, far from diminishing the threat of more instability in Pakistan, will enhance it. The military escalation on Pakistan's border could produce a spillover of militants," Lodhi said. "It will enhance the vulnerability of US-Nato ground supply routes … Protecting these supply lines will overstretch Pakistani forces at present engaged in quashing the Pakistani Taliban.

"The surge could also lead to an influx of more Afghan refugees. It could also provoke a spike in violent reprisals in Pakistan … It is therefore imperative for Islamabad to try to persuade the US to modify its strategy," she said.

Trouble is, Obama, like Gordon Brown, is a man in a hurry with a war to win. One political shove too hard, one cross-border drone attack too many, or another Mumbai-style attack by Pakistani-based groups on an Indian target, and Pakistan could swiftly join the descent into chaos.